The vitamin D synthetic and response mechanisms are phylogenetically ancient. Vitamin D is produced by both single-cell plants and animals. The ancestral vitamin D receptor (VDR) proteins first appeared in worms. By contrast, the role of vitamin D in calcium and bone homeostasis evolved much later in animals with skeletons. This begs two important questions: [1] How did vitamin D serve the organism in advance of skeletal development?; and [2] Is this function still operative in more advanced species, specifically man, today? It is hypothesized that [1] a central function of the vitamin D system is local, not blood-borne, in nature, designed to be active in epithelial barrier protection (i.e. exclusion of invaders from the host) and [2] this function is not vestigial, remaining active in advanced vertebrates, including our own species. In pursuit of the hypothesis 'that the vitamin D hormone (1,25(OH)2D3) is a locally-produced and locally-active factor that acts to modify barrier function in vivo', this research program will employ state-of-the-art in vivo and in vitro molecular technologies in transgenic mouse models that address the following experimental questions and associated hypotheses that have particular relevance to human health and disease. First, does diminished local production of the 1,25(OH)2D3 result in diminished epithelial barrier protection? It is hypothesized that animals harboring targeted disruption of the gene that encodes the enzyme which makes the hormone will be more susceptible to gut invasion by noxious chemicals and infectious agents. Second, considering the millions of humans that suffer from lack of adequate vitamin D nutrition, how does vitamin D insufficiency affect barrier integrity? It is theorized that vitamin D insufficiency will hamper barrier integrity in vivo. And third, if the vitamin D hormone is over-produced at sites of active inflammation, as it is in patients with Crohn's disease, what are the consequences of its over-production? It is proposed that local overproduction of 1,25(OH)2D3 has a dual function to quell inflammatory responses and to preserve mucosal integrity. It is anticipated that 1,25(OH)2D3 synthesized at barrier sites will be shown to amplify barrier protection, providing an explanation for the extra-renal synthesis of the 1,25(OH)2D3 at sites of potential pathogen invasion. It is anticipated that the experiments planned here will: 1] uncover the ancient action of the vitamin D hormone in the preservation of a healthy separation between the mammalian host and its environment; and 2] provide insight in how best to use these barrier preserving actions in humans when infectious or inflammatory disease threatens barrier integrity.